Radiotherapy apparatus delivers a beam of high-energy radiation (typically in the MeV range, apt to damage tumour cells) which is directed towards a tumour site (or other lesion) in a collimated and controlled manner. Thus, the lateral extent of the beam is limited by collimating elements so as to match a pattern determined in a predetermined treatment plan, such as the external profile of the tumour or a subsection of it. The direction of the beam is also varied, so that the tumour is irradiated from multiple directions, thereby reducing the dose delivered to tissue surrounding the tumour site. The treatment is also delivered in “fractions”, i.e. individual fractional doses delivered at intervals of (for example) a day, which add up to a total dose to be delivered; delivering the dose in fractions alleviates the side-effects on the healthy tissue surrounding the tumour site.
Typically, a treatment plan will be drawn up prior to delivery of the first fraction, which will detail the beam shapes, directions, and intensity/duration of a number of beam segments that together will form the first fraction. These beam segments are designed to, collectively, deposit a three-dimensional dose distribution in the tumour which corresponds to that prescribed by a clinician, and which both generally minimises the dose delivered to non-tumour areas, and remains within upper dose limits for certain designated sensitive areas of the patient. This is a challenging problem, and the treatment plan is generally arrived at by an iterative process carried out on a computational device.
It is now fairly common for radiotherapy apparatus to have a detector, usually attached to the gantry, located opposite the high-energy radiation source and positioned so as to detect the radiation beam after it has passed through the patient. Such “portal imagers” usually comprise a flat-panel defector (in the form of an electronic portal image detector, or “EPID”) which can create an image of the treatment beam as attenuated by the patient; from this image and from a priori knowledge of the beam that was delivered, information as to the distribution of radiation fluences passing through the patient can be arrived at. Once the sequence of instantaneous fluence patterns have been brought together after a fraction is complete, they can be used in conjunction with anatomical information acquired prior to or during treatment (for example a planning CT scan or intra-fraction MR image(s)) to estimate the three-dimensional dose pattern or dose equivalent that was delivered during that fraction. A significant computational effort is involved, so the calculation is usually done after the fraction is complete, using all of the observed fluence patterns from the fraction, to allow a post-fraction QA check of the dose delivery.